Multiple-station turret punch machines can provide up to 72 different punch stations for use in conjunction with a like number of opposing dies. In such a machine, each punch station operates as a punch set holder for a removable punch set that includes a centrally-disposed punch surrounded by a punch guide and biased by a punch spring. Even with the flexibility afforded by a 72-station machine, the operator may wish to change some or all of the punch set and die combinations from time to time. For instance, the operator may wish to utilize a different punch tip shape or size in a punch set. It is desirable to minimize the time required, and make it simple, to change the punch and die components, so that down time on the punch machine is minimized.
In a punching operation, after the punch tip strikes the workpiece, the punched workpiece surface will tend to catch, and hence follow, the punch tip as it retracts. The stripper plate has an opening in which the punch tip fits snugly yet can axially move freely through the opening. In use, the punch guide is oriented with the stripper plate flush against the workpiece surface. When the punch tip retracts from the workpiece at the end of a punching operation, the edges of the workpiece around the punch hole will be prevented by the stripper plate from following the retracting punch tip.
Certain punch guide configurations incorporate the stripper plate as an integral part of the punch guide itself. However, since the size and shape of the stripper plate hole must coincide closely with that of the punch tip, each punch guide of the nature is limited to use with a matching punch. Consequently, it may be inconvenient to interchange punch and die combinations, since the operator must change not only the die and punch, but the punch guide as well.
Another configuration uses a flattened metal clip or the like to retain a removable stripper plate at the end of a punch guide. This allows the use of a number of different punches with each guide, since only the stripper plate needs to be changed to accommodate a new punch. These spring clip structures, however, have not provided completely satisfactory performance. Often, the workpiece surface will have a thin coating of oil or other fluid. When the stripper plate meets the workpiece surface, a suction may be created. When this occurs, the stripper plate may be pulled out of place and damage may result to the workpiece and to the stripper plate. Down time may also be a problem. Finally, these clips tend to weaken with continued use, potentially aggravating the noted problems.
Another configuration is shown in commonly assigned U.S. Pat. No. 4,092,888, which depicts a punch guide assembly using a resilient, flat retaining ring to retain a removable stripper plate.
In still another configuration, commonly assigned U.S. Pat. No. 4,248,111 depicts a punch guide assembly that employs stripper plate holding tabs which are on clips attached parallel to the axis of the punch guide.
A further configuration is shown in commonly assigned U.S. Pat. No. 4,446,767, wherein a locking ring 13 is fitted in matching circumferential grooves in a stripper plate 22 in the punch guide sleeve 11. The free ends 31 of the locking ring 13 include tabs 32 that can be spread apart and locked into position by the locking ring expansion lock 14. Although installation and removal of the stripper plate is relatively simple, it involves handling the loose ring 13 and the cap screw used as the locking ring expansion lock 14. In some cases, the manipulation of these small parts may be more difficult and time consuming than is preferred.
Other mechanisms for retaining stripper plates on punch guides are described in U.K. patent specification 1 251 843 and U.S. Pat. No. 3,079,824, to Schoft, U.S. Pat. No. 3,540,339, to Killaly, U.S. Pat. No. 4,947,718, to Whistler, and U.S. Pat. No. 4,989,484, to Johnson et al.
In the U.K. patent specification, an O-ring 25 in annular mating grooves in the punch guide sleeve and the stripper plate provides a snap fit of the stripper plate 21 to the punch guide 18. The '824 patent also illustrates a snap ring 38 for retaining the stripper plate 36 in position. Such snap rings may neither be strong enough nor reliable enough to securely lock the stripper plate in position.
The '339 patent employs retainers 60 and spring-mounted elements 56 for engagement against V-shaped annular detent grooves 57 in the edge of the stripper plate 62. Attachment and removal involves loosening the retainers 60 and moving them aside to snap-in or unsnap the spring-loaded catches 56 in or out of the grooves. The redundant attachment mechanism requires additional steps in releasing or locking the stripper plate.
The '718 patent shows the use of spring clips 194 attached to the punch guide and engaging internally-threaded flanges 196 in the stripper plate 46 in a fashion similar to the '111 patent. Such spring clips may not be strong enough for large diameter punch sets and may weaken with repeated use.
The '484 patent discloses a complex locking ring 80 with positioning springs 90, 92 located within a groove and positioned between a pair of diametrically-opposed pins 94, 96 that engage other pins, such as 98, to hold the ring 80 in the lock position. The stripper plate 74 is held in place in the locking ring by centrally-extending flanges 109 and pin-receiving slots 110. To remove or replace the stripper plate 74, the locking ring 80 is turned about the axis of the punch assembly against the compression of the springs 90, 92, thereby aligning the pin-receiving slots 110 with the pin 74D and allowing the stripper plate 74 to be removed and re-inserted in the fashion of a bayonet-lock mechanism. Then, by pushing the stripper plate in and depressing the release pin 112, the springs 90, 92 rotate the locking ring 80 on the box 76 so the flanges 109 cover the pins 74D, thereby holding the stripper plate 74 securely in place on the end of the punch assembly 10. This rotatable retaining ring and stripper plate assembly includes a number of small parts, such as pins and springs, that add complexity and fabrication costs. Moreover, the stripper plate retaining ring itself is removed when the stripper plate is released, thus potentially risking its loss or damage if dropped.
U.S. Pat. No. 6,047,621 discloses another mechanism for removing and replacing a stripper plate from a punch guide. This mechanism includes slides 80 mounted on a punch guide for axial movement along the front end of the guide. The slides 80 are forwardly and rearwardly movable between locked and unlocked positions. Each slide 80 is locked in position by a spring-loaded button assembly 82 extending radially from the guide 14 through an opening 84 in the lower portion of the slide 80. To release the stripper plate 20, the button 82 is pressed inward, thereby releasing the slide 80. The slide is then urged forward, which causes a ball bearing 86 positioned between an upper, recessed inner surface 88 of the slide 80 and a groove 90 in the periphery of the stripper plate 20 to move radially outward. This causes the ball bearing 86 to move out of engagement with the groove 90 in the outer edge of the stripper plate. Thus, to release the stripper plate 20, one must depress the button on each of the slides 80 and then push each of the slides 80 to the front of the guide. This is less than ideal in terms of ease of stripper plate removal and replacement.
U.S. Pat. No. 6,082,516 discloses a stripper plate release mechanism comprising a locking ring with three arcuate grooves having radially-increasing dimension and three locking clips which are circumferentially fixed relative to the locking ring. The locking clips engage the stripper plate. Upon rotation of the locking ring, the locking clips ride in the arcuate grooves which act as camming surfaces and open the locking clips, thereby releasing the stripper plate. As with certain other prior art stripper plate release mechanisms, this mechanism requires a rotation step to release the stripper plate. It would be desirable to provide a stripper plate release mechanism that does not require rotation or substantial manipulation to remove and replace the stripper plate.
Thus, it can be appreciated that, despite the effort put into designing various types of attachment mechanisms for stripper plates, a need still exists for simple, strong, reliable, and inexpensive-to-manufacture stripper plate locking mechanisms that are easy and quick to use in releasing or attaching the stripper plate, and which securely hold the stripper plate when locked.